1. Field of the Invention
The present invention relates to an optical apparatus provided with detecting means for detecting a position of a moving lens which serves a function such as focusing or zooming.
2. Description of the Related Art
In an optical apparatus, such as a silver-halide camera or a video camera, in which a focusing lens is driven by means of a stepping motor, the position of the focusing lens has conventionally been controlled under open-loop control on the basis of the number of driving steps of the stepping motor. In such a conventional example, it is necessary to detect an absolute position of the focusing lens which is used as a reference position when the apparatus is started up, as by turning on a power source. In general, a detecting device for detecting the absolute position of the focusing lens is arranged in such a manner that a detecting mechanism of a non-contact type, such as a photosensor, or that of a contact type, such as a leaf switch, is provided in one position within the moving stroke of the focusing lens. Detection of the absolute position of the focusing lens is performed in such a way that the focusing lens directly turns on and off the photosensor or the leaf switch.
However, the conventional example has a number of disadvantages. For example, if the time required to detect the absolute position which serves as the reference position of the focusing lens is to be minimized, wherever in the moving stroke the focusing lens is currently located, it is desirable to set the absolute position, which serves as the reference position, in the middle position of the moving stroke of the focusing lens. However, the longer the moving stroke of the focusing lens, the longer the time required to detect the absolute position which serves as the reference position.
If the above-described conventional example is applied to the construction of a lens barrel made of a plastics material, the following problem is encountered. In general, plastic lens barrels tend to expand or shrink due to a temperature variation. In the conventional example in which the absolute position which serves as the reference position of the focusing lens is provided at only one position, even if such a plastic lens barrel expands or shrinks due to a temperature variation during an operation of the stepping motor, it is impossible to detect whether a variation has occurred in the state of focus.
In a conventional optical apparatus, a front-lens-group focusing type of lens has generally been adopted in which a first lens group is moved by a helicoid along its optical axis. However, a recent type of optical apparatus utilizes a so-called, inner- or rear-focus type of zoom lens in which a lens group located rearward of a variator lens which serves a magnification-varying lens group is used to perform focusing.
With the inner- or rear-focus type of zoom lens, it is possible to photograph a subject at a closer distance than the front-lens-group focusing type of lens. Further, the inner- or rear-focus type of zoom lens can be easily constructed so that focusing can be continuously performed between infinity and a position immediately before the front surface of the zoom lens, particularly, on the wide-angle side. Various lens types are known in the field of such an inner- or rear-focus type of zoom lens. For the purpose of illustration, FIG. 11 shows one construction example in which a lens group located at its rear end is used for focusing.
The construction example shown in FIG. 11 includes a front lens 101 which is fixed, a variator lens 102, a fixed lens 103 and a focusing lens 104 (which also serves as a compensator). The shown construction example also includes a guide bar 133 for guiding the variator lens 102 while inhibiting rotation thereof, a feed bar 134 for the variator lens 102, a fixed lens barrel 135, iris unit 136 (which, in the shown example, is inserted perpendicularly to the surface of the sheet of FIG. 11), and a stepping motor 137 which serves as a focusing motor. The stepping motor 137 has an output shaft 138 which is externally threaded to move the focusing lens 104. An internally threaded part 139 is meshed with an externally threaded part of the output shaft 138, and is integral with a moving frame 140 of the focusing lens 104.
The shown construction example also includes guide bars 141 and 142 for the moving frame 140 of the focusing lens 104, a rear plate 153 for positioning and retaining the guide bars 141 and 142, a relay holder 144, a zoom monitor 145, a speed reducing unit 146 for the zoom motor 145, a gear 147 fixed to an output shaft 146a of the speed reducing unit 146, and a gear 148 fixed to the feed bar 134 for the variator lens 102 and meshed with the gear 147.
An image sensor 170 is used for the purposes of sensing an image, measuring the luminance thereof, and detecting focus. A microcomputer 171 controls the driving of each of the motors 137 and 145, an autofocus operation based on an output from the image sensor 170, and various other operations.
In the above-described construction, if the stepping motor 137 is driven, the focusing lens 104 is driven to move along the optical axis by the output shaft 138. If the zoom motor 145 is driven, the feed bar 134 is rotated through the gears 147 and 148, thereby moving the variator lens 102 along the optical axis.
FIG. 12 shows the positional relation between the variator lens 102 and the focusing lens 104 in the above-described inner-focus type of zoom lens with respect to each subject distance, and the respective positions of the variator lens 102 and the focusing lens 104 are plotted along the horizontal axis and the vertical axis.
Loci 150 to 153 correspond to, for example, the respective subject distances shown in Table 1.
TABLE 1 ______________________________________ LOCUS SUBJECT DISTANCE ______________________________________ 150 .infin. 151 2 m 152 1 m 153 Immediately Before Lens ______________________________________
The variator lens 102 and the focusing lens 104 are inhibited from assuming a positional relation which corresponds to any point in each of hatched regions 154 and 155 in FIG. 12.
As shown in FIG. 12, the inner-focus type of zoom lens has a variable-focus-like relation, i.e., a relation in which the position at which its focusing lens is to be located with respect to the position of its variator lens varies for each subject distance.
In the field of such a zoom lens, a number of methods have been proposed for controlling the positional relation between the variator lens and the focusing lens during zooming (for example, U.S. Pat. No. 5,027,147). Although detailed description of the methods is omitted herein, they have the following common arrangement. Absolute-position encoders or the like are respectively provided for detecting the absolute position of the variator lens and that of the focusing lens. The position at which the focusing lens is to be located with respect to the position of the variator lens during zooming and the speed at which the focusing lens is to be moved with respect to such position during zooming are determined from position information about each of the variator lens and the focusing lens and map information, such as that shown in FIG. 12, which is separately memorized in a microcomputer.
To detect the respective positions of the variator lens and the focusing lens, a number of methods are available. For example, it is possible to adopt a method which utilizes an arrangement employing a variable resistor. Otherwise, it is also possible to adopt a method which utilizes an arrangement employing a stepping motor as an actuator. In the case of the latter method, a method of continuously counting the number of pulses inputted to the stepping motor (the number of driving steps) may also be used.
According to any of these detecting methods, when an electric power source is turned on, the variator lens and the focusing lens are made to move to their respective predetermined reference positions and pulse counting is started at each of the reference positions, whereby the respective absolution positions of the variator lens and the focusing lens can be detected.